In recent years, attention has been focused on a wet-granulated toner in place of another toner prepared via mechanical pulverization, since the wet-granulated toner is advantageous to introduction of a large amount of wax capable of exhibiting a sharp particle distribution, together with a particle in small size thereof. Examples of the method of manufacturing a toner granulated by a wet process include an emulsion association method, a suspension polymerization, a dispersion polymerization, and also a dissolving suspension method employing polyester or the like having been separately subjected to polycondensation.
It is disclosed that a polymerized toner obtained by an emulsion association method to form toner mother particles via a polymerization process in an aqueous medium has a sharp particle size distribution together with small particle size thereof, since shape and particle size of the toner mother particles can be controlled in a preparation process, and rounded toner having no corner on the surface of each of particles having uniform shape of the toner mother particle is obtained (refer to Patent Document 1, for example).
Since the toner having uniform shape and size as described above is expected to result in high-resolution images, introduction to digital system image formation to form fine dot images of 1200 dpi (dpi represents the number of dots per inch or 2.54 cm), for example, is increasingly discussed.
After forming toner mother particles in an aqueous medium or an organic solvent to prepare a toner mother particle dispersion, toner mother particles are separated from the toner mother particle dispersion employing a separation device represented by a solid-liquid separator like a filtration device to obtain toner granulated via a wet process after adding external additives, if desired. A surfactant and impurities such as free wax particles released from toner mother particles or their decomposed material particles are contained in a dispersion in which the toner mother particles are dispersed. Accordingly, when separating the toner mother particles from the dispersion, washing should be well-conducted in such a manner that these impurities do not remain in the toner mother particles.
In order to remove impurities from the toner mother particles, disclosed is a technique by which washing water is supplied until electrical conductivity of a filtrate reaches not more than the specific value, while the toner mother particles are separated from the toner mother particle dispersion via centrifugal separation to wash the toner mother particles (refer to Patent Document 2, for example).
Further disclosed is a technique by which the toner mother particles are filtrated under applied pressure to remove impurities after a cleaning solution is added into toner mother particles from which an aqueous medium is removed in a vessel equipped with stirring blades and a filter (refer to Patent Document 3, for example).
In order to flush a surfactant, a dispersion stabilizer and an inorganic salt remaining on the surface of each of toner mother particles, also disclosed is a method by which the toner mother particles are washed with deionized water (refer to Patent Document 4, for example).
Further, in order to flush the surfactant, the dispersion stabilizer and the inorganic salt remaining on the toner mother particles, disclosed is a method by which the toner mother particles (slurry) are washed with deionized water having an electrical conductivity of 1 μS/cm until electrical conductivity of a filtrate reaches 2 μS/cm (refer to Patent Document 5, for example).    (Patent Document 1) Japanese Patent O.P.I. Publication No. 2000-214629    (Patent Document 2) Japanese Patent O.P.I. Publication No. 2000-292976    (Patent Document 3) Japanese Patent O.P.I. Publication No. 2001-249490    (Patent Document 4) Japanese Patent O.P.I. Publication No. 2008-233175    (Patent Document 5) Japanese Patent O.P.I. Publication No. 2006-325895